Among a large number of aberrations in optical systems special attention is paid to astigmatism, because this aberration is a typical and very important for all “classical” spectral instruments.
Mirror lenses, used in spectral instruments, do not have an axis of symmetry and besides aberrations, related to conventional centered systems, possess decentering aberration – astigmatism.
Astigmatism is the off-axis beams aberration when the focal points for the meridional and sagittal planes do not match. Astigmatism increases as the relative aperture is increased (decreasing f-number).
At different positions of an observation plane, image of a point has different shapes. As the observation plane is moved away from the optical surface, ellipse is reduced first and then degenerates into a line (perpendicular to the meridional plane) at the meridional focus. At further movement it is transformed again to an ellipse, further – into a circle, then in an ellipse again and in the focal point of sagittal plane it is transformed to the line (perpendicular to the sagittal plane), after that ellipse increases again.
In spectral instruments without astigmatism correction a plane of exit slit and photodetector is located in a plane of meridional plane of focus. This ensures high spectral resolution, but a point light source on the entrance slit is transformed into monochromatic vertical line in a focal plane of the instrument.
In all spectrographs, manufactured by SOL instruments, astigmatism is corrected through the use of aspheric optics. Spectral instruments with correction of astigmatism are called imaging monochromators and spectrographs.
If to an entrance slit of such type of a spectrograph the multiple point light sources spaced across a slit height are directed the spaced in height spectra will be formed in a focal plane.
This allows you to use Imaging spectral instruments (with compensated astigmatism) in multichannel (multi-track) spectroscopy, where the height spaced spectra, obtained in the focal plane, are recorded simultaneously by a matrix photodetector.
Use of the optical scheme with astigmatism compensation can also minimize the losses of light in a case of small size detectors. This is a specific feature for the IR detectors which have, as a rule, very small size of active area in order to reduce noise.
Compensation of astigmatism gives an additional vertical magnification of a spectral instrument, which should be considered when choosing optic fibers and detection systems.
Let us examine a few examples of signal recording with imaging spectrograph using a matrix CCD detector.